Pipelines that extend through saltwater bodies are employed for multiple purposes, often associated with the drilling of oil and gas wells and the production and distribution of the oil and gas produced by the wells. When the pipelines are not buried or otherwise solidly anchored within the saltwater body, they are subjected to dynamic loadings that can eventually cause the pipelines to rupture or otherwise fail. The stresses induced by dynamic loading in the connections of the individual tubular bodies forming the pipeline are of particular concern.
Jointed pipelines that are secured together by the threaded engagement of pin and box connectors at the ends of individual pipe sections are employed in marine riser pipes as well as submerged pipelines and other bottom-to-surface supply lines. The marine riser pipelines are typically constructed of tubular pipe sections that are secured together at their ends by special connectors designed to withstand the destructive effects of the dynamic loading acting on the riser. The dynamic loading of the riser is caused by changing sea currents, changes in surface exerted tension resulting from wind and wave action against a surface support of the riser, and other factors.
The effects of dynamic loading on the connections used in marine applications may be offset, in part, by the use of connectors that have the strength required to withstand the forces imparted by the environment. Conventional riser connections typically employ heavy tool joint type connections that are welded onto the end of the pipe. These connections are very resistant to fatigue damage and often employ a metal-to-metal torque shoulder as an external seal, similar the design employed in a conventional drill pipe tool joint. Connections of this type have a very low stress concentration factor (SCF) and provide an environmental seal that prevents the seawater from contacting the threads in the connection.
The tool joint design is expensive to machine and requires the added step and expense of welding the connector to the pipe. The welding procedure also introduces a potential point of fatigue failure for the dynamically loaded connector. Moreover, as compared with a conventional threaded and coupled connection, the tool joint type connection is extremely heavy, requiring additional surface support.
Conventional threaded and coupled connections have been successfully employed, for short periods, as the outer riser in a marine installation. Such connections cost less than ⅙ the cost of tool joint type connectors. The long-term reliability of these conventional connections in a saltwater environment under dynamic loading conditions, however, is not reasonably predictable. The uncertainty stems from the fact that, when used as risers, the threads of conventional threaded and coupled connectors are exposed to salt water that can accelerate corrosion of the threaded area. The dynamic loading of the riser constantly flexes the connections laterally and imposes cyclical tension and compression stresses in the connected components. Any unengaged pin threads that have been machined into the pipe portion of the connection are exposed to the salt water. These exposed threads concentrate the stresses exerted on the pipe. Adding the effects of corrosion to the cyclical stress concentration unreasonably exposes the connection to a fatigue induced failure that will generally occur in the area of the last full thread formed on the body of the pin member of the connection.
The strength of a standard threaded and coupled connection is optimized by forming the pin threads such that the thread roots “run out” or continue to decrease in depth on the external surface of the pipe until they disappear at a point referred to as the “last scratch.” The final turns of the threads do not make a full depth cut into the pipe body and are not normally intended to be engaged and covered by threads in the mating coupling. From the point of the last full threaded engagement with the box threads to the last scratch of the pin threads, the pin threads in a conventional threaded and coupled connection are exposed to the surrounding environment. The corrosive effects of saltwater in this area of exposure can accelerate the failure of a connector subjected to cyclical, dynamically induced forces.
U.S. patent application Ser. No. 09/679470, cited previously, discloses a connector design in which the normally exposed pin runout threads on fully engaged, threaded and coupled connections are sealed from saltwater to prevent corrosion of the threaded area that can accelerate fatigue-induced failure in dynamically loaded pipelines. The seals may be employed with conventional threaded and coupled connectors permitting the fabrication of pipelines that are inexpensive and lightweight as compared with pipelines constructed with conventional tool joint type connectors.
The external seal portion of the invention of application Ser. No. 09/679470 may be provided by metal-to-metal engagement between the pin and box or by an elastomeric annular seal compressed between the pin and box. The metal-to-metal external seal may be provided by engagement of the face of the box with a shoulder formed on the pin end of a non-upset pipe. The shoulder may also be provided by enlarging the pin outside diameter. Where an annular, elastomeric seal is provided, the seal may be carried on the outer surface of the pin body or may be disposed in the box of the connector.
The enlarged outside diameter of the pin portion of the parent application connector is formed by expanding the pipe wall radially outwardly. A frustoconical pin seal surface is machined externally on the enlarged radial area. When the connector is assembled, the pin seal surface engages a frustoconical box seal surface internally adjacent the end of the box. Engagement of the two seal surfaces forms an external seal that prevents salt water from entering the threaded area of the connection. During the manufacturing process, if the pin seal surface is not properly machined on the expanded body of the pipe, the entire pipe section can be rendered unusable. Similarly, if a properly machined seal becomes defective through subsequent mishandling or usage, the remaining expanded wall of the pipe may not be adequate for reconstructing or repairing the damaged seal, resulting in a loss of the complete pipe joint.